The present invention relates to a drive control method, specifically a drive control method for a heavy duty drive, in particular a heavy duty drive of a vertical mill designed to crush brittle materials, for example cement raw materials, as well as a control device for implementing the method and a corresponding drive system for a vertical mill that operates according to the method.
Vertical mills of the type mentioned above with a grinding plate rotating about the vertical and grinding rollers above the grinding plate tend to be subject to significant mechanical oscillation, as in simple terms a vertical mill is an oscillatory system in the form of a damped two-mass oscillator. The first mass includes the grinding plate and all the units moved with the grinding plate and the second mass is the rotor of the driving motor. The connection between these two masses is present in the form of the drivetrain, in other words at least one gearbox included in the drivetrain, both of which function in the manner of a torsion spring in the oscillatory system. The system is made to oscillate jerkily, as well as for short periods or even for longer periods as a result of continuous, low-frequency load changes due to the grinding process as well as random alternating stresses due to the grinding process. The resulting forces and torques can become so significant that the grinding process has to be stopped in order to avoid damage to the drivetrain, in particular specifically to the electric motor and/or gearbox or the system as a whole.
In order to minimize such oscillations until now the operator of the mill had to configure the process parameters, in other words in particular a contact pressure of the grinding rollers, a formula for the material to be ground and quantities of grinding aids to be added, in such a manner that the oscillations stimulated remain below a critical level. However this results in undesirable restrictions in process design which have a negative impact in many areas. The range of products that can be produced using the ground material obtained, the effectiveness of the mill, the energy input required and cost efficiency are among the factors affected.
With this in mind and because of increasing demands relating to availability, efficiency and total cost of ownership, the design and arrangement of the electrical and mechanical components of a drive system and of the respective drivetrain of a heavy duty drive, in particular a vertical mill, are of increasing importance.
Currently drive systems with a gearbox and at least one electric motor in the form of an asynchronous motor, preferably a slip ring rotor, as well as at least one frequency converter supplying the at least one electric motor, present a preferred solution for vertical mills. With this the mill gearboxs are frequently embodied in practice as variants of bevel gear or helical gear planetary gearboxs. The task of the gearbox is to absorb the axial grinding forces and transmit them to the platform as well as to convert the rotational speed and torque.
In practice the control of such a drive system for a vertical mill is essentially faced with the following problems:
In order to be able to insure optimum process management, a first, apparently insignificant task of the drive is to insure the prespecified rotational speed of the grinding plate. As the process torque required at the grinding plate fluctuates, rotational speed control is essential.
The load fluctuations and oscillation stimuli acting on the drive mechanism are characterized by pulse loads, as result for example when the grinding rollers pass over coarse materials being ground, stochastic loads from the grinding process, periodic stimuli from the gearbox and mill kinematics and a varying grinding roller contact pressure. The interaction of these stress influences results in a complex load cycle, which can even instigate resonant oscillation.
In addition to drivetrain oscillation a non-stable grinding bed, in other words a fluidizing grinding bed or one characterized by waviness, can also result in extreme oscillation states of the mill, in particular mill rumble.
Finally the grinding of natural products makes it largely unpredictable how the grinding process should be set to insure quiet running of the mill. It is therefore always a challenge for the operator in control to set the correct process parameters. Ultimately the drive alone can alleviate an unfavorably set process but it cannot correct it.
The approach proposed here deals with a reduction of the stresses acting on the drivetrain during operation of the vertical mill. Until now attempts have been made to reduce such stresses by using couplings in the drivetrain. However a coupling is known to be an expensive part that is subject to wear and the way in which oscillations are damped by means of a coupling is based on the conversion of oscillation energy to heat, which has a negative impact on the energy balance sheet of the vertical mill. It has also proven from observation of vertical mills in operation that the drivetrain oscillations remain at a very high level even when a coupling is used.